In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these higher densities, there have been, and continue to be, efforts toward scaling down the device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features generally requires high resolution lithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and the film exposed with a radiation source (such as optical light, x-rays, or an electron beam) that illuminates selected areas of the surface through an intervening master template, the mask, forming a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposed portions of the coating become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image or its negative in the remaining coating.
Uniform and consistent resist coating is important to obtaining extremely fine patterns after exposure of the resist. For example, a coating thickness should vary by no more than ±100 Å across the wafer surface and from wafer to wafer. Although spray coating, meniscus coating, roller coating, curtain coating, extrusion coating, plasma deposition, and electrophoresis have all been used to apply resist coatings, spin coating is the usual method.
In a typical spin coating process, a small quantity of resist solution is dripped or sprayed onto a semiconductor substrate. The resist may be applied to the center of the substrate or in a pattern from center to edge. The resist may be applied in a helix pattern, for example, by slowly turning the wafer while scanning a dispense head from center to edge. The resist is initially spread across the surface by spinning the substrate at low speeds, (e.g., 200 rpm for 1 second). Then the spin rate is rapidly ramped up to a final spin speed in the 3000 to 7000 rpm range. The thickness of the final coating can depend on many parameters such as volume of solution dispensed, substrate diameter, resist solution viscosity, spin speed during dispense, rate of acceleration to final spin-speed, and final spin speed. Small changes, caused for example by the evaporation of solvent during spin-speed ramp-up, can significantly affect coating thickness.
Clean conditions must be maintained to avoid defects in the resist coating. The resist should be clean and free of particles above 0.2 μm in diameter. Because the resist is sticky, it can easily entrap airborne particles. Therefore, resist coating should be carried out in a Class-100 or better environment. Defects can also be caused by air bubbles entrapped in the resist.
A common cause of defects and variability in resist coatings is the tendency of resists to dry rapidly and form residues on the dispense head. These residues can occlude the dispense head orifice, affecting the amount and pattern in which the resist is dispensed. In addition, flakes of dried resist and particles that crystalize from the resist solution as it dries may contaminate the resist solution or fall directly onto the substrates.
One way to avoid having resist solution dry at the dispense head is to maintain a steady flow of resist through the dispense head in between applications. This is called dummy dispensing. This method can be effective, but resist solutions are expensive and the amount of wasted resist involved in dummy dispensing cause this method to be prohibitive.
Another approach is to flush the dispense head with solvent between uses. One difficulty with this approach is that solvent in the dispense head may dilute subsequently dispensed resist solution. Diluting the resist solution affects its viscosity and results in variable coating thickness. The dispense head can be flushed with resist solution before dispensing on substrates, but as with dummy dispensing this involves the waste of expensive resist solution. The dispense head can also be submerged in a solvent between uses, with similar consequences.
Another idea is to place the dispense head, between uses, under an atmosphere saturated with solvent. Unfortunately, it is difficult to maintain the correct solvent atmosphere, particularly in a location in which the dispense head can be easily placed and removed. Additionally, the required apparatus is complicated and residues may still form.
Other measures can be taken to reduce the extent to which resist dries on the dispense head. A vacuum suck-back in the resist solution supply line can reduce the amount of resist drying on the dispense head. A non-stick coating can improve the effectiveness of the vacuum suck-back. However, some resist remains in the dispense head and the remaining resist tends to dry very quickly.
Dispense heads may also be constructed so that they can be frequently changed. This approach may be employed to avoid defective coatings, but only at the price of expense, equipment downtime, and inconvenience.
In view of the above, there remains an unsatisfied need for an apparatus and method of dispensing resist that is convenient, uncomplicated, does not waste expensive resist solution, and keeps the dispense head relatively free of residues and contaminates.